Recently, wireless power transfer, in which power is transferred wirelessly without using a metal contact such as a connector, has been placed into practical use. In the wireless power transfer, a power supply apparatus and a power receiving apparatus transmit/receive control signals or the like to exchange information for various control operations such as a notification of required power. For example, a load modulation scheme is used for transmitting/receiving the control signals. Particularly, as a method of sending information from the power receiving apparatus to the power supply apparatus, a method of changing the load on the power receiving apparatus side to transmit data for a power transfer signal sent by the power supply apparatus is known (see Japanese Patent Laid-Open No. 2004-215225 and Japanese Patent Laid-Open No. 2008-206231).
Furthermore, in the recent wireless power transfer, the use of a resonant magnetic coupling scheme has been examined. The resonant magnetic coupling scheme is advantageous in that power transfer is possible in a wide range, compared to an electromagnetic induction scheme where the power supply apparatus and the power receiving apparatus have to be very near. For this reason, in the resonant magnetic coupling scheme, it is also possible to supply power to a plurality of power receiving apparatuses simultaneously.
When performing wireless power transfer, if there is a foreign substance such as a metal fragment near a power supply apparatus, an eddy current is generated in that foreign substance, increasing power consumption. Therefore, in this case, power to be received deteriorates in a power receiving apparatus which receives power supply. Similarly, power is also received in another power receiving apparatus when the other power receiving apparatus gets closer to the power supply apparatus, resulting in the deterioration of power to be received in the power receiving apparatus. That is, the power receiving apparatus cannot discriminate between a case in which there is the foreign substance and a case in which there is the other power receiving apparatus from the received power deterioration, and cannot distinguish between an abnormal state in which power is consumed by the foreign substance and a normal state in which a plurality of power receiving apparatuses share power.
The present invention has been made in consideration of the aforementioned problems, and can specify the cause of a change in received power in wireless power transfer.